Method and system for treating mixed municipal and selected commercial waste

Abstract

A method of treating municipal solid waste comprising the steps of: shredding the waste to a first particle size,—storing the shredded waste in an aerated storage bay ( 14 ) for a first period time; shredding at least some of the waste to a second particle size,—and agitating the shredded waste in a rotating drum ( 19 ) for a second period of time and then outputting the treated waste; wherein biological decomposition under aerobic conditions takes place in the aerated storage bay and the rotating drum.

1 . A method of treating municipal solid waste comprising the steps of:
 shredding the waste to a first particle size;   storing the shredded waste in an aerated storage bay for a first period time;   shredding at least some of the waste to a second particle size; and   agitating the shredded waste in a rotating drum for a second period of time and then outputting the treated waste;   wherein biological decomposition under aerobic conditions takes place in the aerated storage bay and the rotating drum.   
   
   
       2 . A method according to  claim 1 , wherein the first particle size is 100 mm or less. 
   
   
       3 . A method according to  claim 1  or  2 , wherein the second particle size is 50 mm or less. 
   
   
       4 . A method according to  claim 1 , wherein the first period of time is approximately 72 hours. 
   
   
       5 . A method according to  claim 1 , further comprising the step of controlling airflow through the storage bay such that the temperature of the shredded waste is maintained at approximately 50-55° C. 
   
   
       6 . A method according to  claim 1 , further comprising the step of applying a heating cycle and a cooling cycle to the waste during the first period of time;
 wherein the maximum temperature of the heating cycle is approximately 55° C. and the minimum temperature of the cooling cycle is approximately 40° C.   
   
   
       7 . A method according to  claim 1 , wherein the average moisture level in the shredded waste after the first period of time is reduced to around 35% to 40% by weight, by metabolically generated heat. 
   
   
       8 . A method according to  claim 1 , wherein the second period of time is approximately 48 hours. 
   
   
       9 . A method according to  claim 1 , further comprising the step of periodically applying a heating cycle and a cooling cycle to the waste during the second period of time. 
   
   
       10 . A method according to  claim 9 , wherein the heating cycle comprises exposing the waste to a first airflow rate at a time when the rotating drum is stationary; and
 the cooling cycle comprises exposing the waste to a second airflow rate, higher than the first airflow rate, at a time when the rotating drum is rotating.   
   
   
       11 . A method according to  claim 9  or  claim 10 , wherein the duration of each heating cycle and each cooling cycle is controlled such that the temperature of the waste in the rotating drum is maintained between around 40° C. and 55° C. 
   
   
       12 . A method according to  claim 11 , further comprising the step of:
 heating the second airflow to a temperature above the ambient temperature, before it enters the rotating drum;   wherein the second airflow is heated from heat generated from the biological decomposition in the storage bay.   
   
   
       13 . A method according to  claim 1 , wherein the average moisture level in the shredded waste after the second period of time is reduced to around 15% to 25% by weight, by metabolically generated heat. 
   
   
       14 . A method according to  claim 1 , further comprising the step of:
 sorting, before the first shredding step, the municipal solid waste to remove non-combustible material from the waste.   
   
   
       15 . A method according to  claim 1 , further comprising the step of:
 feeding the treated waste from the rotating drum into a rotary dryer for a third period of time and heating the treated waste to a temperature of around 90° C. to reduce the average moisture level in the treated waste to around 15% by weight.   
   
   
       16 . A system for treating municipal waste, the system comprising:
 a first shredder arranged to shred the waste to a first particle size;   an aerated storage bay constructed to aerate the shredded waste;   a second shredder arranged to shred at least some of the stored waste to a second particle size;   a rotary drum arranged to agitate and aerate the waste; and   means for controlling the temperature and the moisture levels in the waste in the storage bay and the rotary drum such that biological decomposition under aerobic conditions takes place in the aerated storage bay and the rotary drum.   
   
   
       17 . A system according to  claim 16 , further comprising a rotary dryer arranged to receive and reduce the moisture content of material output from the rotary drum. 
   
   
       18 . A method of treating municipal solid waste comprising the steps of:
 shredding at least some of the waste to a first particle size; and   agitating the shredded waste in a rotating drum for a period of time and then outputting the treated waste;   wherein biological decomposition under aerobic conditions takes place in the rotating drum.   
   
   
       19 . The method according to  claim 18 , further comprising the step of controlling airflow through the rotating drum such that the temperature of the shredded waste is maintained at approximately 40-55° C. 
   
   
       20 . The method according to any one of  claim 18  or  19 , further comprising the step of applying a heating cycle and a cooling cycle to the waste during the period of time;
 wherein the maximum temperature of the heating cycle is approximately 55° C. and the minimum temperature of the cooling cycle is approximately 40° C.   
   
   
       21 . The method according to  claim 20 , wherein the heating cycle comprises exposing the waste to a first airflow rate at a time when the rotating drum is stationary; and
 the cooling cycle comprises exposing the waste to a second airflow rate, higher than the first airflow rate, at a time when the rotating drum is rotating.   
   
   
       22 . The method according to  claim 21 , wherein the duration of each heating cycle and each cooling cycle is controlled such that the temperature of the waste in the rotating drum is maintained between around 40° C. and 55° C. 
   
   
       23 . The method according to  claim 22 , further comprising the step of:
 heating the second airflow to a temperature above the ambient temperature, before it enters the rotating drum.   
   
   
       24 . A system for treating municipal waste, the system comprising:
 a first shredder arranged to shred at least some of the waste to a first particle size;   a rotary drum arranged to agitate and aerate the waste; and   means for controlling the temperature and the moisture levels in the waste in the rotary drum such that biological decomposition under aerobic conditions takes place in the rotary drum.   
   
   
       25 . A system according to  claim 24 , further comprising a rotary dryer arranged to receive and reduce the moisture content of material output from the rotary drum.